A major area of interest in agriculture, especially in light of the determination of the complete nucleotide sequences of a number of plant genomes, is the targeted alteration of genome sequences. In particular, the ability to convert endogenous plant sequences would facilitate numerous applications such as, for example, the optimization of crop traits affecting nutritional value, yield, stress tolerance, pathogen resistance, and resistance to agrochemicals and/or the adaptation of plants for use as biological factories for the production of pharmaceutical compounds or industrial chemicals.
In eukaryotes, attempts have been made to alter genomic sequences in cultured cells by taking advantage of the natural phenomenon of homologous recombination. See, for example, Capecchi (1989) Science 244:1288-1292; U.S. Pat. Nos. 6,528,313 and 6,528,314. If a polynucleotide has sufficient homology to the genomic region containing the sequence to be altered, it is possible for part or all of the sequence of the polynucleotide to replace the genomic sequence by homologous recombination. However, the frequency of homologous recombination under these circumstances is extremely low. Moreover, the frequency of insertion of the exogenous polynucleotide at genomic locations that lack sequence homology exceeds the frequency of homologous recombination by several orders of magnitude.
The introduction of a double-stranded break into genomic DNA, in the region of the genome bearing homology to an exogenous polynucleotide, has been shown to stimulate homologous recombination at this site by several thousand-fold in cultured cells. Rouet et al. (1994)Mol. Cell. Biol. 14:8096-8106; Choulika et al. (1995) Mol. Cell. Biol. 15:1968-1973; Donoho et al. (1998)Mol. Cell. Biol. 18:4070-4078. See also Johnson et al. (2001) Biochem. Soc. Trans. 29:196-201; and Yanez et al. (1998) Gene Therapy 5:149-159. In these methods, DNA cleavage in the desired genomic region was accomplished by inserting a recognition site for a meganuclease (i.e., an endonuclease whose recognition sequence is so large that it does not occur, or occurs only rarely, in the genome of interest) into the desired genomic region.
However, meganuclease cleavage-stimulated homologous recombination relies on either the fortuitous presence of, or the directed insertion of, a suitable meganuclease recognition site in the vicinity of the genomic region to be altered. Since meganuclease recognition sites are rare (or nonexistent) in a typical plant genome, and insertion of a suitable meganuclease recognition site is plagued with the same difficulties as associated with other genomic alterations, these methods are not broadly applicable.
Thus, there remain needs for compositions and methods for targeted alteration of sequences in any plant genome and for compositions and methods for targeted introduction of exogenous sequences into a genome.